A New Aluminium Piston.
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THE USE of aluminium for pistons is gaining in favour, it being generally recognized that pistons of this type possess important advantages for use in high-speed internal-combustion engines. Amongst these advantages is a reduction in reciprocating weight, and high heat conductivity, although in the case of aluminium pistons of ordinary design these may be negatived by the excessive clearances which must be allowed. If these clearances be not given the piston tends to seize up in the cylinder, and this tendency is accentuated by distortion, which may occur when the piston is first, set in motion. To obviate the drawbacks of the ordinary aluminium piston, the Aerolite Piston Co., Ltd.; 36, North Audley Street, London, VT.1, have recently introduced a new patent type of piston for which many advantages are claimed.
The general design of the piston can be seen from the accompanying illustration. It is• of . the dumb-bell form and has an upper portion intended to carry the ordinary rings, and a portion at. the bottom of the skirt provided with a groove for accommodating a bearing ring, the depth of the groove being such that the ring bears on the wall of the cylinder without appreciable clearance. To allow for expansion of the piston several short vertical slots are provided on each working face ; none of them, however, is located near the plane of the gudgeon pin. In order to allow for full expansion of the bottom ring, the groove is deepened, say, from two to five onethousandths of an inch. The ring is thus free to float in the plane of the gudgeon pin, so that it takes the thrust of the cylinder wall in that direction, but, it is prevented from turning in its groove by..an aluminium dowel.
At the top of the waist of the piston a series of small holes is drilled at regular intervals in a.diagonal direction. Any excess of lubricating oil which gets past the bearing ring passes through these holes and impinges on the inside of the crown of the piston, and thus. materia,117 helps to cool it.
The gudgeon pin is usually made a fairly tight fit in the bosses, but is left free to rotate when it is bedded in, end play being prevented by means of copper and almnunum pastilles. This method the company have found more satisfactory than the Use of Jarass or steel hushes, as these instals havee-a different co-efficient OF expansion as compared with In -designing the new piston the .makers have endeavoured to secure as ample a feint as possible, and all -unnecessary webs, which tend to accentuate distortion, have been avoided. The crown of the piston has been made somewhat thicker than is really necessary, the chief :0bject!of . this construction being to assist in the -dissipation of surplus heat.